No start

Hi all:
I received a mower from a friend of mine that he wanted to see what I could do with it. The wheels were are in terrible shape but my friend said don't worry about the wheels. The mower, 4hp Briggs and Stratton quantum) ran but it was surging a lot. I took the carb off, to it to my friend and got a rebuild kit for the carb. Well, I did what I could and now, the engine wont run at all. I cleaned it up the best I could. Now let me ask you this...when I tried to start the engine I did not have the air filter housing and filter installed along with the carb intake gasket. Do I have to have that housing with the filter installed? There is a rubber tube that comes off of the filter housing that goes to (I believe) the breather. Does that have to be connected for it to start? I checked for spark and I have it. The plug is dry. No gas got to the cylinder. Maybe I didn't clean it good enough? Like I said, it did start and surged before I tore it down. Now, after reassembling everything, it wont start.

Thanks
Holis